1. Technical Field
The present invention relates to a senseless Maximum Power Point Tracking (hereinafter, referred to as “MPPT”) control apparatus of a photovoltaic power generation system and a method thereof, and more particularly, to such a senseless MPPT control apparatus of a photovoltaic power generation system and a method thereof in which a maximum output point of a solar battery cell can be tracked by using only one feedback current flowing into a load, thus always producing an optimal output, and feedback factors are reduced to one, thus further simplifying the construction of a control circuit and minimizing tracking control failure.
2. Background Art
In general, a MPPT control method of a photovoltaic power generation system can be largely divided into a power comparison method and a constant voltage control method.
In the power comparison method of the MPPT control method, as shown in FIGS. 1 and 2, a solar battery cell 1 made of crystalline silicon (single crystalline or polycrystalline), amorphous silicon, compound semiconductor or the like and configured to transform photoelectromotive force into electric energy generates current and voltage as analog signals. A current transformer 2 and a voltage detector 4 detect the current and voltage, respectively, through sampling in real-time (S1). First and second A/D converters 5, 6 convert the current and voltage into current and voltage of digital signals and output the converted current and voltage to a power calculation unit 7. The power calculation unit 7 calculates power based on the current and voltage and stores the results (S2).
An old cell power detector 8 and a new cell power detector 9 detect an old cell power OP and a new cell power NP, respectively, from the signals of the power calculation unit 7, and transfer the detected old and new cell powers OP and NP to a new and old cell power comparator 10. The new and old cell power comparator 10 compares the received old and new cell powers OP and NP to determine whether the new cell power NP is greater than the old cell power OP (S3).
If, as a result of the comparison, the new cell power is greater than the old cell power (NP>OP; Yes in step S3), the new cell power NP and the old cell power OP are transferred to a first new and old voltage comparator 11 that compares a new cell voltage NV and an old cell voltage OV at that state. If, as a result of the comparison, the new cell power is smaller than the old cell power (NP<OP; No in step S3), the new cell power NP and the old cell power OP are transferred to a second new and old voltage comparator 12 that compares a new cell voltage NV and an old cell voltage OV at that state.
The first and second new and old voltage comparators 11, 12 compare the new cell voltage NV and the old cell voltage OV at their states (S4, S5), and output the comparison results to a voltage adder 13 and a voltage subtractor 14, respectively.
The voltage adder 13 adds a voltage shift ΔV to a voltage value Vd, which was measured and stored one sampling earlier than a value that is currently input, depending on the output values of the first and second new and old voltage comparators 11, 12 (S6). For example, if the NP is grater than the OP (Yes in step S3) and the NV is greater than the OV (Yes in step S4), the voltage adder 13 adds the voltage shift ΔV to the voltage value Vd, which was measured and stored one sampling earlier than a currently input value, and outputs the resulting value to a reference voltage generator 15. If the NP is smaller than the OP (No in step S3) and the NV is smaller than the OV (No in step S5), the voltage adder 13 adds the voltage shift ΔV to the voltage value Vd, which was measured and stored one sampling earlier than a currently input value, and outputs the resulting value to the reference voltage generator 15.
Further, the voltage subtractor 14 subtracts the voltage shift ΔV from the voltage value Vd, which was measured and stored one sampling earlier than a currently input value, depending on the output values of the first and second new and old voltage comparators 11, 12 (S7). For example, if the NP is greater than the OP (Yes in step S3) and the NV is smaller than the OV (No in step S4), the voltage subtractor 14 subtracts the voltage shift ΔV from the voltage value Vd, which was measured and stored one sampling earlier than a currently input value, and outputs the resulting value to the reference voltage generator 15. If the NP is smaller than the OP (No in step S3) and the NV is greater than the OV (Yes in step S5), the voltage subtractor 14 subtracts the voltage shift ΔV from the voltage value Vd, which was measured and stored one sampling earlier than a currently input value, and outputs the resulting value to the reference voltage generator 15.
The reference voltage generator 15 generates a new reference voltage based on the voltage (S8). A subtractor 16 receives the reference voltage from the reference voltage generator 15, subtracts an output voltage of the solar battery cell 1, which is output to the voltage detector 15, from the reference voltage, to calculate an error value (S9), and outputs the error value to a PI controller 17.
The PI controller 17 outputs a control signal, corresponding to the error value, to a PWM signal generator 3 (S10).
A pulse width of the control signal, output from the PWM signal generator 3, is converted accordingly in real-time and then output to a DC/DC converter 18 (S11). Thus, a maximum output point can be tracked in response to shift in voltage and current of the solar battery cell 1 in which a DC voltage, output from the DC/DC converter 18 for supplying voltage from the solar battery cell 1 to each load, is shifted every moment. The tracked maximum output point can be supplied in real-time (S12).
Meanwhile, FIG. 3 shows a power-voltage characteristic curve of a solar battery cell. Assuming that a point where MPPT control begins is 0 (P0, V0), a point 1 is P1, V1, a point 2 is P2, V2, and a point 3 is P3, V3, V is increased (+) and P is also increased (+) at the point 0 in order to track the maximum power. In the case of a course 1 from the point 0 to the point 1 and then a course 2 from the point 1 to the point 2, V is increased (+), but P is decreased (−). Thus, the voltage shift ΔV has to be decreased (−) in order to track the maximum power.
Further, if the course 2 changes to a course 3 from the point 2 to the point 3, V is decreased (−), but P is increased (+). However, for maximum value tracking, the voltage shift ΔV must be decreased (−). After the course 3 (an opposite side after the maximum point), V is decreased (−) and P is also decreased (−). Thus, the voltage shift ΔV must be increased (+) (a course 4).
(where a control factor is voltage V and power P=current I)
An algorithm with respect to the conventional MPPT control scheme as described above can be expressed in the following table 1.
TABLE 1CourseVPΔV0 −> 1+++1 −> 2+−−2 −> 3−+−3 −> 4−−+
In this case, all the remaining constituent elements other than the current transformer 2, the voltage detector 4 and the DC/DC converter 18 can be built in one processor, although they are separately shown in the drawing.
However, the power comparison method described above is adapted to operate at the maximum output point based on an output power and voltage of the solar battery cell. Thus, two sensors, that is, a current transformer and a voltage detector are required at the output terminal of the solar battery cell. Further, two A/D converters are also required at the processor in order to calculate a current and voltage, input as an analog signal from the current transformer and the voltage detector, based on the algorithm as shown in FIG. 2. In addition, a calculation process is also relatively complicated since such calculation is performed based on two inputs.
In other words, the power comparison method of the conventional MPPT control method is of a type in which an increase and decrease are compared between an output voltage and current of the solar battery cell and a feedback power and voltage thereof in order to track a maximum output always. This control method has a complicated control algorithm and is problematic in that there is a significant danger of tracking control failure.
Meanwhile, the constant voltage control method has a simplified control algorithm since only an output voltage of the solar battery cell is feedbacked. Thus, a danger of tracking control failure can be minimized and control stability can be maximized. However, an optimal output cannot be always output because an output voltage of the solar battery cell is fixed.
Accordingly, it is a fact that the conventional MPPT control method has a specific shortcoming in each control method.